1. Field of the Invention
The present invention relates to a charged particle therapy system, and more particularly to a charged particle therapy system having a range modulation wheel (hereinafter abbreviated to “RMW”), in which a charged particle beam, such as a proton or carbon ion beam, is irradiated to a tumor for treatment.
2. Description of the Related Art
A known charged particle therapy system comprises a charged particle beam generator, an ion beam transport system, and a rotating irradiation apparatus. The charged particle beam generator includes a synchrotron (or a cyclotron) as an accelerator. A charged particle beam accelerated to setting energy by the synchrotron reaches the rotating irradiation apparatus through the ion beam transport system (first ion beam transport system). The rotating irradiation apparatus comprises an irradiation-apparatus ion beam transport system (second ion beam transport system), an irradiation field forming apparatus, and a rotating apparatus (rotating gantry) for rotating the second ion beam transport system and the irradiation field forming apparatus as an integral structure. The ion beam is irradiated to a tumor, e.g., a cancer, in the body of a patient from the irradiation field forming apparatus through the second ion beam transport system.
The irradiation field forming apparatus has the functions of not only shaping the ion beam extracted from the charged particle beam generator in match with a three-dimensional shape of the tumor as an irradiation target, thereby to form an irradiation field, but also adjusting the dose in the irradiation field. As one of methods for irradiating the ion beam in the desired dose in match with the shape of the irradiation target, a double scattering method is known which makes uniform the dose of the ion beam with two types of scatterers arranged at a distance between them in the axial direction of the ion beam by utilizing the fact that the dose of an ion beam having passed a scatterer has a nearly Gaussian distribution (see, e.g., FIG. 36 in page 2081 of Non-Patent Reference 1; “REVIEW OF SCIENTIFIC INSTRUMENTS”, VOLUME 64, NUMBER 8 (AUGUST 1993), P2074-2086). As another irradiation method, there is also known a wobbler method for making uniform the dose of the ion beam by using two scanning magnets (see, e.g., FIG. 41 in page 2084 of Non-Patent Reference 1).
In those irradiation methods, a Bragg peak spreading-out (SOBP) device is employed to make uniform the dose of the ion beam in the depth direction of the tumor in the patient body (i.e., in the direction of irradiation of the ion beam). Examples of the SOBP device include a ridge filter (see, e.g., FIG. 31 in page 2078 and FIG. 41 in page 2084 of Non-Patent Reference 1) and an RWM (see, e.g., FIG. 30 in page 2077 of Non-Patent Reference 1).
The RWM is rotatably installed on a path of the ion beam within the irradiation field forming apparatus. The RWM has a plurality of blades extending radially from a rotary shaft, and distal ends of the blades are joined to a cylindrical member. The cylindrical member is concentric to the rotary shaft. Each blade has multiple steps each having a different thickness and arranged successively in the circumferential direction of the RWM. Each step having a different thickness is extended from the rotary shaft to the cylindrical member. The ion beam propagating in the irradiation field forming apparatus passes those steps when passing through the rotating RMW. Therefore, the ion beam having passed the RWM has a plurality of energy components corresponding to the thickness of each of the steps through which the ion beam has passed. As a result, the dose of the ion beam is made uniform in the depth direction of the tumor in the patient body.